Messenger from Earth reaches Mercury orbit (photos)

The spacecraft Messenger is on a mission to get close and stay close to the innermost planet in the solar system to help NASA learn more about rocky planets--including our own.

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Messenger approaches Mercury

After three flybys of Mercury over the last three years, the Messenger spacecraft is poised to settle into the first-ever orbit by a manmade object of the planet closest to the Sun. The goal of the mission is to provide scientists with data on Mercury unparalleled since the Mariner 10 spacecraft passed by three decades ago. And the more we understand about Mercury, NASA says, the more we'll understand about how the other rocky planets in the solar system--Venus, Mars, and of course, Earth--formed and evolved.

The insertion into orbit starts at about 9 p.m. ET on March 17. From that point on, Messenger will fly a 12-hour orbit at a minimum altitude of about 124 miles. The spacecraft's science instruments will be turned on and checked out starting on March 24.

First orbital images

The first orbital image to come from Messenger, as it passes over Mercury's south pole, will be of the area outlined here in yellow. It will include a portion of the planet surface that hasn't yet been documented by a spacecraft. NASA says that first planned image will be captured on March 29, with Messenger set to acquire a total of 364 images in the first six hours. During its "commissioning" phase--a checkout of the spacecraft and its instruments in the initial days in orbit--Messenger will acquire a total of 1,549 images.

The primary science phase begins April 4 and will last about one year; during that time, Messenger is expected to gather more than 75,000 images.

Caloris basin

Mercury isn't really blue and gold. "In general," says NASA, "in light visible to the human eye, Mercury's surface shows only very subtle color variations." The space agency explains how it gets from subtle to bold: when images from all 11 narrow-band color filters of the Wide Angle Camera in the Mercury Dual Imaging System "are statistically compared and contrasted, these subtle color variations can be greatly enhanced, resulting in extremely colorful representations of Mercury's surface."

In this mosaic of what NASA calls the "eastern limb" of Mercury, from the January 2008 flyby (the first of the three by Messenger), the large gold-hued circular area is the Caloris basin, notable for its volcanic plains. The basin is about 960 miles in diameter, and Messenger was passing by at a distance of about 8,000 miles.

October 2008 flyby

This enhanced-color mosaic comes from the second flyby of Mercury, which took place in October 2008, as Messenger was flying away from the planet--hence the widening in the series of images. The narrowest images, at left, were taken about nine minutes after the spacecraft's nearest approach, while the widest were take at the 15-minute mark, when Messenger was twice the distance away. The small blue crater at upper right (a "young, rayed crater"--the rays fade over time with exposure to the harsh space environment) has since been named Dominici. Click here for a larger view of the mosaic.

The name "Messenger," by the way, stands for a much longer phrase: MErcury Surface, Space ENvironment, GEochemistry, and Ranging.

Orbiting Mercury

This graphic shows two views of Messenger's orbit, one from the perspective of Earth, the other from the perspective of the sun. At least once every Mercury year (about 88 Earth days), the spacecraft will have to execute "propulsive maneuvers" to keep its minimum altitude below 310 miles. After entering orbit this week, Messenger will have just 9.5 percent of the usable propellant it carried when it left Earth in August 2004, but that will be plenty for the orbital corrections it will need to make, NASA says.

Science instruments

These are the science instruments that Messenger is carrying. Because of the mass of propellant that the spacecraft needed to carry, the payload mass was limited to 110 pounds. In addition, the instruments had to be mounted so that they could see Mercury but not be exposed to the sun, and they need to be maintained within an acceptable temperature range in what NASA calls a "harsh thermal environment." (Messenger does have a sun shade.)

Basho and Tolstoj

This mosaic of images, captured by the Narrow Angle Camera of the Mercury Dual Imaging System in January 2008, shows a pair of impact craters, Basho and Tolstoj. As with Dominici on an earlier slide here, the rays around Basho suggest that the crater is relatively young. Of the "broad annulus" of dark material in the Tolstoj crater region, NASA asks, "could it be that this dark material is similar to Basho's and was also excavated from below Mercury's surface when the Tolstoj-forming impact occurred? Messenger's orbital mission will provide a variety of data needed to investigate this question and many others aimed at understanding the composition of Mercury's crust and its variation both laterally and with depth."

Rachmaninoff crater

The recently named Rachmaninoff crater, which has a diameter of 180 miles, is "an intriguing double-ring basin" that has piqued NASA's interest "because of its fresh appearance, its distinctively colored interior plains, and the extensional troughs on its floor." This mosaic image was acquired by the third flyby, in September 2009. Click here for a closer look at Rachmaninoff.

Naming craters

This portion of a global mosaic of Mercury's surface shows 10 craters that received their names in March 2010, when the International Astronomical Union conferred its blessing. Craters on Mercury can't be named for just anyone; rather, they must honor "deceased artists, musicians, painters, and authors who have made outstanding or fundamental contributions to their field and have been recognized as art historically significant figures for more than 50 years." Click here for a closer view of all those craters.

Geddes crater

The crater Geddes, named for Irish stained glass and graphic artist Wilhelmina Geddes, has a "complex geological history that appears to have involved both tectonic deformation and volcanism," NASA says. The image at bottom right is from the Mariner 10 mission in 1974-75. The orange hue in the enhanced-color image at top right, captured by Messenger's Wide Angle Camera in October 2008, is suggestive of "explosive volcanism." The image at left was also captured in October 2008, but with the Narrow Angle Camera. Geddes has a diameter of 50 miles. Click here to get up close with Geddes.

Earth, far away

In May 2010, Messenger took a look back toward home--that brighter spot at lower left is Earth, and to its right, the smaller bright spot is the moon. The image was captured with the Wide Angle Camera when Messenger was about 114 million miles from Earth. Click here for a larger view.